The optical time domain reflectometer (OTDR) is an instrument which is used for measuring the attenuation and other characteristics of a fiber optic cable. The OTDR sends light pulses down an optical fiber and measures the reflected and backscattered light. The principal advantages of the OTDR method are that the fiber cable does not have to be cut nor do both ends of the fiber cable have to be accessed.
The fiber characteristics and features along the cable will cause portions of the light pulse to be backscattered and reflected back to the OTDR. The OTDR has a receiver stage which includes a photodetector. The photodetector detects the backscattered and reflected light and converts it into electrical signals. The OTDR can include a boxcar integrator circuit which integrates the electrical signals for improving the signal-to-noise ratio (SNR). The boxcar circuit can also include an amplifier stage. The OTDR uses the signals from the boxcar integrator to generate a waveform which shows the characteristics and features of the fiber as a function of time, but displays it on a monitor as a function of distance along the length of the fiber cable.
The waveform produced by an OTDR typically has three types of features. The first feature comprises an initial pulse, which results from reflection of the input light pulse at the OTDR-fiber optic connection. The second feature of the waveform is an exponentially decaying plot which corresponds to the Rayleigh backscattering that occurs as the light pulse propagates down the length of the fiber cable. Rayleigh backscattering is caused by imperfections in the fiber such as impurities, air bubbles, or moisture. The third feature comprises discrete pulses or peaks which are the result of Fresnel reflection. Fresnel reflection typically occurs at connection points because of the glass-air-glass interface. Fresnel reflection can also result from physical breaks in the fiber.
The level of the reflected light resulting from Fresnel reflection can be up to 4% of the initial light pulse which is injected by the OTDR into the fiber cable. The Rayleigh backscattered light, on the other hand, produces a very low level signal. The level of the Rayleigh backscattered light is inversely proportional to the wavelength of the light pulse raised to the fourth power. For example, as the wavelength of the input light pulse is increased from 850 nanometers (nm) to 1550 nm, the signal level due to backscatter decreases dramatically. Therefore, to accurately reproduce the fiber optic cable features, the OTDR must have the capability to detect and process signals over a large dynamic range. The dynamic range of signals typically processed by an OTDR can be 60 decibels (dB) in terms of optical power.
There are problems in digitizing a signal having such a high dynamic range. First, the range of signal levels over a high dynamic range signal can saturate the analog-to-digital converter and/or the amplifier in the receiver stage of the OTDR. Known OTDRs typically amplify the signal prior to analog-to-digital conversion in order to improve the signal to noise ratio (S/N). Secondly, to cover the typical signal range of an OTDR, i.e. 60 dB, the analog-to-digital converter must have a resolution of at least 1,000,000 steps. Known 20-bit A/D converters can have a 1,048,576 step resolution; but, 20 bit A/D converters are costly. In addition, the conversion time for a 20-bit A/D is usually too long for effective data acquisition as required in real time applications such as an OTDR system.